This invention relates to the use of hazy zinc oxide in the preparation of photovoltaic devices. In particular, this invention relates to the use of a hazy zinc oxide composition in a single conductive layer or in combination with other conductor layers in the preparation of thin film solar cells and modules.
In view of the significant costs associated with the early single-crystal silicon solar cells, a substantial amount of research has been devoted to the use of alternative semiconductor materials. One widely adopted alternative has been the use of thin films of silicon-hydrogen alloys (TFS) in either microcrystalline or amorphous form and including other components such as hydrogen, nitrogen, carbon, germanium, tin, etc. In addition, a variety of binary, ternary and quaternary semiconductor compounds have been widely employed in the preparation of photovoltaic devices; combinations of particular interest include II/VI, III/V, I/III/VI and II/IV/V compounds.
A particularly valuable ternary semiconductor compound for use in photovoltaic devices is the I/III/VI compound copper indium diselenide (CuInSe.sub.2 or CIS). This compound has high optical absorption coefficients over much of the solar spectrum and can absorb 90% of the useful solar spectrum in a thickness of less than 1 .mu.m. Exemplary procedures for preparation of photovoltaic devices including CIS semiconductor layers are reported in U.S. Pat. No. 4,335,266 to Mickelsen et al., U.S. Pat. No. 4,581,108 to Kapur et al. and U.S. Pat. No. 4,465,575 to Love et al., the disclosures of which are hereby incorporated by reference. A method for preparation of a CIS film by DC sputtering first of copper and then of indium using separate cathodes, followed by heating of the composite film in the presence of a gas containing selenium, is taught in U.S. Pat. No. 4,798,660 to Ermer et al. the disclosure of which is also hereby incorporated by reference. A presently preferred method for fabrication of a CIS film is taught in co-pending U.S. application No. 07/273,616 filed Nov. 17, 1988 and assigned to the same assignee as this application, the disclosure of which is also hereby incorporated by reference.
The basic cell structure of a CIS solar cell as disclosed in co-pending U.S. application No. 07/273,616 comprises a ZnO transparent front electrode, a thin CdS interfacial layer, a CuInSe.sub.2 absorber layer and a Mo back electrode. The efficiency of such devices and the resulting power density of large-area CIS modules is strongly dependent on a large number of factors, including the effect of the front electrode with respect to cell photocurrent density, module resistive losses and patterning quality.
Integrated thin film solar modules place stringent requirements on the properties of the transparent conductor layers. In thin film cell structures the transparent conductor layer typically acts both as an optical transmitter and as a conducting electrode. As an optical transmitter, the transparent conductor must have high optical transmission over the response spectrum of the absorber semiconductor film, for example, CIS. CIS has an optical bandgap of approximately 1 eV, so that it is essential that the transparent conductor fully transmit the longer-wavelength "red" solar spectrum, requiring that plasma absorption effects be minimal. CIS solar cell structures are typically heterojunction structures capable of being responsive to the shorter-wavelength "blue" solar spectrum, requiring that the transparent conductor have a wide bandgap so as to transmit the shorter-wavelength light. A third optical requirement often desired of the transparent conductor is a suitable index of optical refraction and surface structure so as to minimize optical reflection. Thin film module designs typically lack front current collecting grids; therefore, the sheet resistance of the transparent conductor electrode is substantially more important in modules than in individual gridded solar cells. In particular, the sheet resistance plays an important role in determining the maximum power output achievable for a module. For high power CIS modules in particular, photocurrent densities on the order of 40 mA/cm.sup.2 demand high conductance electrodes to minimize resistive losses.
Thin film solar cell structures sometimes address the stringent optical and electrical requirements on the transparent conductors by adding additional optical layers into the cell structure. For example, a common CIS cell structure incorporates a transparent conductor optimized for its conductance and optical clarity, and utilizes a separate anti-reflection layer such as SiO.sub.x to optimize cell photocurrents. The use of separate transparent conductor and anti-reflection layers adds additional complexity and cost to the module fabrication process.
An additional factor which must be considered in optimizing transparent electrodes for solar cell module fabrication is the effect of the electrode film on the electrical interconnects between cells of the module. A low resistance contact from the front electrode of one cell to the back electrode of an adjacent cell is essential. Moreover, the transparent electrode layer should be easily patterned.
ZnO is well-suited for use as the transparent conducting electrode in a thin film solar module. Its optical bandgap of approximately 3.3 eV is wide enough to transmit the shorter-wavelength "blue" solar spectrum. ZnO can be deposited at relatively low temperatures with low lattice damage, for example by the chemical vapor deposition process disclosed in U.S. Pat. No. 4,751,149, the disclosure of which is also hereby incorporated by reference. Low-temperature, low-damage deposition is particularly important for efficient photovoltaic structures incorporating compound semiconductor films with volatile components. ZnO has electronic properties such as electron affinity and work function suitable for making efficient heterojunctions with compound semiconductors, such as CIS. The electrical conduction of ZnO films can be controlled by composition control or by the addition of extrinsic dopants such as H, Al, Ga, In or B. In practical use in thin film solar modules, the optical transmission and the electrical conductance must be simultaneously optimized. It is necessary to balance longer-wavelength plasma absorption due to charged free carriers against the higher conductance of high free carrier density. In CIS solar cells and modules, transparent conductor layers of materials such as ZnO doped with B, Al and/or H have heretofore been reported to provide a reasonable balance of transparency and conductivity. Typically, conductivity of these layers is maximized while minimizing optical transmission losses so as to maximize conversion efficiency of the cell or module to make the use thereof economically feasible.
It is known that the efficiency of photovoltaic devices comprising TFS semiconductor films may be increased through the exploitation of light trapping by internal reflection of scattered radiation in the semiconductor films themselves. Deckman et al. provide an optical enhancement model for describing the improved infrared absorption probability in TFS textured semiconducting sheets relative to flat sheets [Deckman, H.W. et al., "Optical Enhancement of Solar Cells," 17 IEEE Photovo Haic Specialist Conference" 955-960 (984)]. The magnitude of the photogenerated current is reportedly determined by several factors, including the optical properties of the electrical contacts. Increases in photogenerated current observed are described as comprising both optical enhancement and antireflection components.
Conversion efficiency of TFS solar cells has been found to have a close relation to the grain size of tin oxide transparent electrodes employed in glass substrate/transparent electrode/p-i-n/back electrode-type solar cells [Iida, N. et al., "Efficiency of the H Solar Cell and Grain Size of SnO.sub.2 Transparent Conductive film," IEEE Electron Device Letters EDL-4(3); 157-9 (1983)]. The observed increase in photocurrent density at short-circuit J.sub.sc and decrease reflectivity were both believed to be caused by light trapping in the large grain SnO.sub.2 film and adjacent TFS semiconductor layer. The conversion efficiency and short-circuit current of cells comprising such "milky tin oxide on glass" (MTG) substrates were subsequently reported as comparable to those of conventional thicker i-layer cells with double layer transparent conductors comprising indium tin oxide layers [Iida, H. et al., "A Milky Tin Oxide on Glass (MTG) Substrate Thin Undoped Layer p-i-n Amorphous Silicon Solar Cell with Improved Stability and Relatively High Efficiency," IEEE Electron Device Letters EDL-5(3): 65-7 (1984)].
It has further been reported that the selection of the side to which a texture is applied can have an important effect on the photovoltaic characteristics in TFS solar cells [Iida, H. et al., "The Structure of Natively Textured SnO.sub.2 Film and Its Application to an Optical Confinement-Type a-Si:H Solar Cell," IEEE Transactions on Electron Devices ED-34(2): 271-6 (1987)]. Application of a textured tin oxide film at the incident light side reportedly gave a more suitable optical confinement effect than a texture treatment at the back side electrode, resulting in an improvement in collection efficiency over the wide wavelength range from 0.3 to 0.8 .mu.m.
U.S. Pat. No. 4,532,537 to Kane suggests a photodetector comprising a light transmissive electrical contact having a textured surface and a semiconductor body overlying the textured surface of the light transmissive electrical contact, wherein the surface texture of the electrical contact is characterized in that it has a dominant peak-to-valley roughness greater than about 100 nanometers. According to the patent, a layer of tin oxide or indium tin oxide is deposited by chemical vapor deposition (CVD) onto a heated substrate; the higher the temperature at which the deposition occurs, the greater the texture, provided that the temperature is less than the temperature at which the substrate softens.
U.S. Pat. No. 4,732,621 to Murata et al. describes a process in which a uniform transparent conductive oxide layer comprising indium tin oxide (ITO) doped with 5 weight-% SnO.sub.x is deposited to a predetermined thickness and then etched using a concentrated chemical etchant in order to make the surface of the transparent conductor rough. The thus-treated ITO layer is described as having a decreased, substantially constant reflectance throughout the visible light range.
Various U.S. patents describe TFS solar cells in which indium and/or tin oxide transparent conductor layers having a specified structure are incorporated. Thus, U.S. Pat. No. 4,746,372 to Tajika al. suggests a conductive transparent film which has micro-columns or fine crystals formed at least on one surfaoe thereof at 500 to 2000 Anqstroms in height. U.S. Pat. No. 4,694,116 describes a transparent electrode comprising two superimposed layers which are separately deposited on a transparent substrate, the average grain diameter of the second layer being smaller than the average grain diameter of the crystal grains of the first layer; any sharp protrusions at the surface of the first layer are effectively rounded by the provision of the second layer thereon formed of crystal grains with a relatively small average grain diameter. U.S. Pat. No. 4,500,743 to Hayashi et al. describes a transparent electrode wherein the average grain diameter of the surface ranges from 0.1 to 2.5 .mu.m. U.S. Pat. No. 4,689,438 to Fukatsu et al. describes a transparent conductor layer wherein the surface is textured in the form of a large number of triangular or quadrangular pyramids having a height of about 1,000 to 3,000 Angstroms and a pitch of about 1,000 to 3,000 Angstroms.
It is an object of the present invention to provide improved transparent conductors for use in the preparation of a variety of photovoltaic devices, such as solar cells and modules.
It is a further object of the present invention to provide photovoltaic devices with optimum solar energy conversion efficiencies.
It is yet another object of the present invention to provide procedures for the efficient and reproducible preparation of optimal transparent conductors for use in any given photodetector system.